1. Field of the Invention
The invention relates to a device for short-circuit protection of an electromechanical contactor, in particular in order to protect the contacts of the contactor from being welded by a short-circuit load, with a current-limiting mechanism. The current-limiting mechanism is connected in series with the contactor and comprises a contact configuration having a fixed contact and a moving contact as well as an arcing chamber associated with them.
A current-limiting mechanism for carrying the current at least at times is known from German patents DE 40 40 359 C2 and DE 41 10 335 C2. Furthermore, a configuration for disconnecting branches of a low-voltage cable network in the event of a short-circuit is known from European patent EP 0 691 046 B1. There, loads are connected to phase conductors and to an associated neutral conductor in the branch, a switching element for switching on and/or off is connected upstream of the load or loads, and additional switching elements for short-circuit disconnection are provided in the phase conductor and/or neutral conductor. A bridging circuit with switches is connected to the switching elements, and a single circuit breaker is provided only in the phase conductor. In this case, the bridging circuit on the load side of the single circuit breaker carries a portion of the short-circuit current past the switch, with the bridging circuit containing a bridging current path with a resistance whose resistance value is between 25 and 50% of the value of the smallest load circuit impedance which can be expected, and via which the magnetically driven switch in the bridging circuit is connected to the neutral conductor. Such a configuration is designed for specific short-circuit protection requirements.
Electromechanical contactors are used for frequent switching of operational currents. Since contactor magnetic drives react to switching-on and switching-off commands with a time delay of 10 to 20 ms, contactors are not suitable for providing protection devices, which act quickly with them, for example in the event of a short-circuit. Thus, in order to limit damaging effects of short-circuits, contactors are electrically connected in series with a circuit breaker or with fuses. Depending on their short-circuit response, an approved combination of a contactor and circuit breaker or contactor and fuse (in accordance with IEC 947-4) is subdivided into coordination type 1, 2 or 3.
With coordination type 1, the contactor must be unusable after a short-circuit, without any danger originating from it. In coordination type 2, slight contact welding, which can be broken open by using a screwdriver, may be present, so that the switching function can be reproduced with little effort. In coordination type 3, there must be no adverse effect on operation of the contactor whatsoever.
The aim of switchgear manufacturers is to provide coordination type 2 or 3 for contactor applications. Contact materials which have particular resistance to welding, such as AgSnO2, are selected for this purpose, and the current which is passed in the event of a short-circuit is limited to sufficiently low values by matching of the switch-contactor or fuse-contactor equipment combination.
The latter measure has the undesirable consequence that the contactors cannot be used in accordance with their rated-current specification and that the operating currents of the equipment combination must be reduced to half the contactor rated operating current, or even less.
Switch-contactor equipment combinations of coordination type 3 (no limitation to operation) are nowadays offered for relatively small sizes (for example In=A) as a complete unit (manufacturer""s designation integral contactor). The bridge contacts of the contactor are in this case positively opened by a magnetic release in the event of a short-circuit and, at the same time, the short-circuit current is limited to safe values by a switching mechanism which limits powerful currents.
However, this method cannot be directly transferred to contactors with relatively high rated currents (100-400 A). It, furthermore, would necessitate considerable modification of the basic design of the contactor.
The object of the present invention is to provide an optimized current-limiting device for present-day contactors which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which allows the optimization without the implementation of structural and design measures. In particular, the novel specification is intended to allow operation for coordination type 3, as mentioned above, without any limit to the rated operating current.
With the above and other objects in view there is provided, in accordance with the invention, a device for short-circuit protection of an electromechanical contactor, in particular for protecting the contacts of the contactor from being welded by a short-circuit load. The device comprises:
a current-limiting mechanism connected in series with an electromechanical contactor, the current-limiting mechanism comprising a contact configuration having a fixed contact, a movable contact, and an arcing chamber associated therewith;
the current limiting mechanism defining a first bridging circuit with a bridging contact, a second bridging circuit, and arc runners or running rails;
the current-limiting mechanism being configured to first close the bridging contact of the first bridging circuit for reducing a current load on the contactor, and to then establish a bridging function with a switching arc and via the second bridging circuit, whereby, in a further course of a disconnection process, the switching arc commutates from the bridging contact onto the arc runners and runs into the arcing chamber, for interrupting the short-circuit.
In other words, the short-circuit protection is provided by a current-limiting mechanism of a type which, in a first stage, closes the bridging contact of a bridging current path in order to reduce the current load on the contactor and whose bridging function, in a second stage, is taken over by the switching arc which, in the further course of the disconnection process, runs into an arcing chamber and in the process interrupts the short-circuit.
In accordance with an added feature of the invention, the bridging contact includes an electrically conductive striker pin (which acts on the movable contact) of a magnetic release and a consumable element of a support carrying the movable contact.
In accordance with an additional feature of the invention, the contactor is connected to associated connecting terminals electrically connected in parallel with the bridging circuits.
In accordance with another feature of the invention, the current-limiting mechanism has main contacts electrically connected in parallel with the bridging circuits, and electrically connected in series with the contactor which is connected to the associated connecting terminals.
In accordance with a further feature of the invention, the short-circuit current is conducted through a magnet coil of the magnetic release until a time at which the bridging contact closes.
In accordance with again a further feature of the invention, closing times of between 1 and 2 ms are achievable by the assembly.
In accordance with again an added feature of the invention, once contact has been made by the bridging contact, the movable contact and the fixed contact of a main current path of the current-limiting mechanism are disconnected, and the short-circuit current commutates onto a current path defined by the first bridging circuit.
In accordance with again an additional feature of the invention, a release magnet armature is configured to trip after commutation of the short-circuit current.
In accordance with again another feature of the invention, the current-limiting mechanism is configured to allow the arc to commutate from the bridging contact onto the fixed contact guide rail when the short-circuit current commutates from the first bridging current path onto the second bridging current path.
In accordance with yet an added feature of the invention, the first bridging current path through the bridging contact carries electric current only during short-circuit disconnection.
In accordance with a concomitant feature of the invention, a current-monitoring device is connected in the first bridging circuit for tripping the current-limiting device if an electric current of a predetermined magnitude and time duration is exceeded. In a preferred embodiment, the current-monitoring device is an electro-thermal bimetallic release.
In other words, the bridging contact is advantageously formed by the magnetic release, which acts on the movable contact, at the striking point. The short-circuit current flows through the magnet coil of the magnetic release until the time at which the bridging contact closes, which allows closing times of between 1 and 2 ms to be achieved. Immediately after the bridging contact makes contact, the movable contact and the fixed contact of the main current path are disconnected as a result of the opening impact, and the short-circuit current commutates onto the bridging current path with a corresponding reduction in the current flowed on the contactor and on the release coil. The latter leads to the tripping of the release magnet armature, thus resulting in the arc commutating, if this has not already happened, from the bridging contact onto the fixed contact guide rail.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device for short-circuit protection, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.